Magnetic disc memory drives comprise a plurality of magnetic memory discs axially spaced on a common shaft and driven at constant rotational speed. In one type of disc memory drive a single surface of one disc is provided with concentric tracks of servo code. The surfaces of the remaining discs have concentric tracks of recorded data. Corresponding tracks on the disc surfaces define track cylinders. An armstack supports magnetic heads in corresponding positions adjacent the surfaces of the discs. The head adjacent the servo code may be called the servo head and the other heads the data heads. A motor driven carriage coupled to the armstack moves the magnetic heads across the tracks or supports the heads at a selected track cylinder.
Control systems for such a disc memory drive have included dedicated servo arrangements for track cylinder seeking and track following using only a dedicated servo head. This assumes that track centering of the dedicated servo head results in track centering of the data heads which is unlikely when the track width is very small.
The dedicated servo approach limits track recording density because of mechanical tolerances in track recording, in magnetic head mounting on the armstack and in the changing physical relationships among the differing parts, for example, armstack tilt, or disc axis tilt, or both.
In sampled servo disc drives, instead of employing a disc surface and a magnetic head dedicated to servo code, the servo code is recorded on each disc usually in equally angularly spaced sectors in each track, interspersed with the data code. The head on each disc is then used in the control system loop for both track seeking and track following over the adjacent disc surfaces as well as for reading and/or writing.
In some of these systems, as referenced in U.S. Pat. No. 4,217,612 entitled "Servo System for Track Accessing and Track Following In a Disc Drive", filed Nov. 27, 1978, using the same drive system for both track accessing, or track seeking, and track following modes, after the addressed track has been reached, switching from the track seeking mode to the track following mode takes place. Switching is undesirable since it frequently results in switching transients or drive force discontinuities producing magnetic head oscillation, interfering with the speed and accuracy of positioning of the magnetic head.
Improvement was achieved, according to the teachings of U.S. Pat. No. 4,217,612, in the provision of a servo system which couples position error signals and position signals to a summing junction at the input to a compensator circuit in the control loop of the servo. With this arrangement there is no switching between track seeking and track following modes of operation which eliminates the switching transients causing the detrimental oscillations of the magnetic heads but control loop saturation is a problem.
Systems of the type referred to above and others, as described in U.S. Pat. No. 3,936,876, entitled "Rotatable Data Storage Apparatus with Track Selection Actuator Having Multiple Velocities", filed Jan. 21, 1974, U.S. Pat. No. 4,439,800, entitled "Servo Control of Seek Operations in Magnetic Disc Drive", filed Apr. 24, 1981, and U.S. Pat. No. 4,134,053, entitled "Method and Means for Capturing Magnetic Tracks", filed Oct. 26, 1976, during track accessing or track seeking, sometimes provide signals to the carriage motor or actuator control system, defining a velocity profile, to cause the magnetic head to be accelerated over a first predetermined distance to a selected velocity, to be moved at the selected velocity over a second predetermined distance, and to be retarded over the remaining distance so that the velocity of the head as it approaches the addressed or selected track is reduced to the value called the capture velocity that it may be captured and controlled by a track follower control for track centering. The track follower remains connected throughout the seek operation, however, it is not functional since its affect in the seek operation is swamped or overpowered by the seek power until the head velocity is reduced to track follower capture velocity.
Microprocessors or computers are used to provide velocity profiles for the control systems. Speed regulation in some systems has been provided using tachometer feedback in a closed control loop. In other systems as described in U.S. Pat. No. 4,333,117, entitled "Disc Head Arm Position Controller Using Digital Velocity Sensing", filed June 30, 1980, head velocity is measured using the transit time between adjacent tracks of the servo code. In still other systems such as described in U.S. Pat. No. 3,936,876, referred to below, the integral of the coil current and the derivative of the position error signal are used to provide a measure of head velocity. Such loop stabilizing feedback techniques add to system complexity and cost.